In a conventional type shadow mask for a color picture tube, a front surface of a cathode ray tube has employed a curved surface (dome type), wherein a surface of a shadow mask is slightly curved and welded at a frame. Material for such a conventional type shadow mask can be deformed so as to fit a dome shape. Therefore, its press formability and shape freezing performance so as to maintain a pressed shape are attached importance.
Watching a curved surface cathode ray tube taxes the eyes since external lights are reflected on the curved surface irregularly. To resolve the drawback, a flat surface cathode ray tube has been long felt needed. In the shadow mask type, it would be difficult to flatten a curved surface employed to a conventional cathode ray tube.
Therefore, it has been desired material suitable for a completely flat shadow mask employed to a conventional shadow mask type cathode tube.
In a shadow mask type cathode ray tube, although several ideas have been recommended in order to flatten a surface of a shadow mask, one of the most preferable industrial methods is an expanded shadow mask method in which tensile force is applied in an upper-lower direction.
In accordance with a predetermined dot pattern, penetrated holes are provided at a shadow mask material by an etching method. While a frame of the shadow mask material is pressurized in a direction opposite to an expanded direction, the shadow mask material is bonded on the frame by welding and so on. Pressurized force applied to the frame is removed so that tensile force is occurred by reversing force of the frame. In order to avoid for occurring secondary electron, thermal reflection, rust and other baneful elements, a black treatment is operated at 450 to 470° C. for 10 to 20 minutes.
Necessary characteristics of material for such an expanded shadow mask are an excellent anti-etching characteristic, sufficient strength against tensile force and a high temperature creep characteristic in the case of the black treatment. In the black treatment, while tensile force caused by the reversion force of the frame is loaded on the shadow mask material, a thermal treatment is operated. A creep phenomenon is occurred on the shadow mask material by thermal energy and the tensile force so that the shadow mask material is elongated. In the case of a television set wherein a shadow mask of which a creep elongation is large and the tensile force is reduced is installed in a cathode ray tube, the shadow mask would be vibrated largely when a sound volume becomes large. It is a reason why color gapping is occurred.
In a conventional art, it has been considered some ideas, that is, a reduction of pressurized force applied to a frame, an improvement of rigidness of the shadow mask material and a cushion material provided at a portion for supporting a speaker and a cathode ray tube. However, at the present stage, a sufficient effect can not be obtained.
Japanese Patent No. 2548133 discloses steel plate material for an aperture grill type shadow mask comprising low carbon steel plate with nitrogen of 40 to 100 ppm and manganese of 0.20 to 0.60 wt %, wherein a creep elongation in the black treatment is reduced by utilizing the low carbon steel plate with high strength and high temperature creep characteristic. A radius of a solution atomic of nitrogen (N) is much smaller than that of a solution atomic of iron (Fe) so that nitrogen can be formed as immersion type solid solution in crystal lattice of iron. It is so called as Cottrell atmosphere. At a high temperature such as a black treatment temperature, diffusion speed of nitrogen is fast in iron so that a movement of dislocation creep can be controlled by forming cloud like Cottrell atmosphere in which nitrogen atoms are gathered around the dislocation creep. Therefore, it has been considered that the creep elongation is reduced.
Japanese Patent Laid-Open Publication No. 11-222628 discloses a technical idea as similar as Japanese Patent No. 2548133. Japanese Patent Laid-Open Publication No. 11-222628 recites a steel plate further including nitrogen of 100 to 170 ppm and manganese of 0.10 to 0.60 wt % in order to avoid for overlapping with a claimed scope of Japanese Patent No. 2548133.
A subject of the present invention is to provide a color picture tube corresponding to the completely flat cathode ray tube described above and to resolve a color gapping problem by improving the rigidness of the shadow mask material, that is, a reduction of a creep elongation of the expanded slot type shadow mask.